Method of programming a nonvolatile memory cell by reverse biasing a diode steering element to set a storage element

ABSTRACT

A method of programming a nonvolatile memory cell. The nonvolatile memory cell includes a diode steering element in series with a carbon storage element The method includes providing a first voltage to the nonvolatile memory cell. The first voltage reverse biases the diode steering element. The carbon storage element sets to a lower resistivity state.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a Continuation-In-Part of U.S. application Ser. No.12/318,021, filed Dec. 19, 2008, entitled “Programming A Memory CellWith A Diode In Series By Applying Reverse Bias” incorporated herein byreference in its entirety. This application is also aContinuation-In-Part of U.S. application Ser. No. 12/339,338, filed Dec.19, 2008, entitled “Capacitive Discharge Method for Writing to an NVM,”which claims the benefit of provisional application 61/076,553 filed onJun. 27, 2008, which is incorporated herein by reference in itsentirety. This application is also a Continuation-In-Part of U.S.application Ser. No. 12/339,313, filed Dec. 19, 2008, entitled “ReverseSet with Current Limit for Non-Volatile Storage,” which claims thebenefit of provisional application 61/076,553 filed on Jun. 27, 2008,which is incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates generally to the field of memory devicesand more specifically to the field of nonvolatile memory devicescontaining diode steering elements.

Nonvolatile memory arrays maintain their data even when power to thedevice is turned off. In one-time-programmable arrays, each memory cellis formed in an initial unprogrammed state, and can be converted to aprogrammed state. This change is permanent, and such cells are noterasable. In other types of memories, the memory cells are erasable, andcan be rewritten many times.

Cells may also vary in the number of data states each cell can achieve.A data state may be stored by altering some characteristic of the cellwhich can be detected, such as current flowing through the cell under agiven applied voltage or the threshold voltage of a transistor withinthe cell. A data state is a distinct value of the cell, such as a data‘0’ or a data ‘1’. In a one diode, one resistivity switching materialmemory cell, the state of the memory cell is programmed by forwardbiasing the diode. However, the current that surges through the diode ofa memory cell when the resistivity switching material changes stateduring forward biasing can be difficult to control.

SUMMARY

One embodiment relates to method of programming a nonvolatile memorycell. The nonvolatile memory cell includes a diode steering element inseries with a carbon storage element The method includes providing afirst voltage to the nonvolatile memory cell. The first voltage reversebiases the diode steering element. The carbon storage element sets to alower resistivity state.

Another embodiment relates to a method of programming a nonvolatilememory cell array. The nonvolatile memory cell array includes aplurality of X lines, a plurality of Y lines, and a plurality of memorycells. Each memory cell includes a diode steering element in series witha carbon storage element. Each memory cell can be electrically connectedbetween one of the plurality of X lines and one of the plurality of Ylines. A selected X line of the plurality of X lines associated with atleast one selected memory cell of the plurality of memory cells isdriven to a X line select voltage. At least one selected Y line of theplurality of Y lines associated with the at least one selected memorycell is driven to a Y line select voltage. The X line select voltage andthe Y line select voltage are configured such that a diode steeringelement of the at least one selected memory cell is reverse biased. Theat least one selected Y line of the plurality of Y lines is isolatedbefore a carbon storage element of the at least one selected memory cellis completely set to a lower resistivity state. The at least oneselected Y line is held in an isolated state until the carbon storageelement of the at least one selected memory cell is completely set tothe lower resistivity state.

Another embodiment relates to a method of programming a nonvolatilememory cell. A temperature of at least one memory cell or a chipcomprising the at least one memory cell is sensed. The at least onememory cell includes a steering element and a storage element. A settingvoltage is selected based on the sensed temperature. The setting voltageis provided to the at least one memory cell, such that the steeringelement is reverse biased and the storage element is set to a lowerresistivity state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a memory cell with a temporary resistor inaccordance with a representative embodiment.

FIG. 2 is a schematic of a memory cell with multiple temporary resistorsin accordance with a representative embodiment.

FIG. 3 is a top view of a memory cell located on the same chip as atemporary resistor in accordance with a representative embodiment.

FIG. 4 is a top view of a memory cell located on the same circuit boardas a temporary resistor in accordance with a representative embodiment.

FIG. 5 is a top view of a memory cell separate from a temporary resistorin accordance with a representative embodiment.

FIG. 6 is a graph of setting a memory cell with a 800 kΩ resistor inaccordance with a representative embodiment.

FIG. 7 is a graph of setting a memory cell with a 400 kΩ resistor inaccordance with a representative embodiment.

FIG. 8 is a graph of setting a memory cell with a 200 kΩ resistor inaccordance with a representative embodiment.

FIG. 9 is a graph of setting a memory cell with a 100 kΩ resistor inaccordance with a representative embodiment.

FIG. 10 is a graph comparing reading a memory cell by applying a forwardbias in multiple states in accordance with a representative embodiment.

FIG. 11 is a graph depicting an I-V profile of a diode in a memory cellafter being programmed with a 100 kΩ resistor in accordance with arepresentative embodiment.

FIG. 12 is a graph depicting an I-V profile of setting a fresh memorycell with a 800 kΩ resistor in accordance with a representativeembodiment.

FIG. 13 is a graph depicting an I-V profile of setting a second freshmemory cell with a 100 kΩ resistor in accordance with a representativeembodiment.

FIG. 14 is a graph comparing readings from a first fresh memory cellprogrammed with a 800 kΩ resistor and a second fresh memory cellprogrammed with a 100 kΩ resistor in accordance with a representativeembodiment.

FIG. 15 is a schematic of a carbon storage element test circuit inaccordance with a representative embodiment.

FIG. 16 is a graph of pulse voltage versus time for a forward bias ofthe carbon storage element test circuit of FIG. 15 in accordance with arepresentative embodiment.

FIG. 17 is a graph of read current at 2V variability for ten forwardbiased carbon storage element memory cells in accordance with arepresentative embodiment.

FIG. 18 is a current-voltage graph of a diode of a memory cell inaccordance with a representative embodiment.

FIG. 19 is a perspective view of a memory cell in accordance with arepresentative embodiment.

FIG. 20 is a flowchart of programming the memory cell of FIG. 19 inaccordance with a representative embodiment.

FIG. 21 is a graph of the read current at 2V and programming voltage formultiple programming cycles of the memory cell of FIG. 19 in accordancewith a representative embodiment.

FIG. 22 is a circuit diagram of a memory cell array in accordance with arepresentative embodiment.

FIG. 23 is a flowchart of programming the memory cell array of FIG. 22in accordance with a representative embodiment.

FIG. 24 is a graph of a bit line voltage versus time in accordance witha representative embodiment.

FIG. 25 is a schematic diagram of a capacitive discharge circuit inaccordance with a representative embodiment.

FIG. 26 is a timing diagram that explains various embodiments foroperating the circuit of FIG. 25 in accordance with a representativeembodiment.

FIG. 27 is a schematic diagram of another embodiment of a circuit thatcan be used to set a memory cell using the capacitive discharge inaccordance with a representative embodiment.

FIG. 28 is a flow chart describing the operation of the circuit of FIG.27 in accordance with a representative embodiment.

FIG. 29 is a current-voltage graph of a diode of a memory cell atvarious temperatures in accordance with a representative embodiment.

FIG. 30 is a circuit of a temperature-based algorithmic bias selector inaccordance with a representative embodiment.

FIG. 31 is a circuit of a temperature-based look-up bias selector inaccordance with a representative embodiment.

DETAILED DESCRIPTION

A structure for and method of programming a nonvolatile memory cell byreverse biasing a diode steering element to set a storage element aredescribed. In the following description, for purposes of explanation,numerous specific details are set forth to provide a thoroughunderstanding of exemplary embodiments of the invention. It will beevident, however, to one skilled in the art that the invention may bepracticed without these specific details. The drawings are not to scale.In other instances, well-known structures and devices are shown insimplified form to facilitate description of the exemplary embodiments.

Referring to FIG. 1, a schematic of a memory cell 110 with a temporaryresistor 120 in accordance with a representative embodiment is shown.The memory cell 110 includes a diode 130 and a resistivity switchingmaterial element 140. Alternatively, the memory cell 110 can includemultiple diodes; for example, a diode can be located on both sides ofthe resistivity switching material element. In one embodiment, a seconddiode is also included in the memory cell. The memory cell 110 is partof a memory cell array in a memory device. During programming, thetemporary resistor 120 and a voltage source 160 are electricallyconnected to the memory cell 110 by a switch 150. The memory cell 110and the voltage source 160 are grounded together.

The diode 130 can be any suitable diode, such as a semiconductor diode.Examples of a semiconductor diode include p-n and p-i-n semiconductordiodes formed in single crystal, polycrystalline or amorphoussemiconductor material, such as silicon, germanium, silicon-germanium orcompound semiconductors, such as Group III-V or II-VI semiconductors.Alternatively, the diode 130 can be a metal insulator metal (MIM) diode,a metal insulator-insulator metal (MIIM) diode, tunneling diodes, or anytwo terminal non-linear conducting device.

The resistivity switching material element 140 is preferably aresistivity switching material selected from an antifuse dielectric,fuse, diode and antifuse dielectric arranged in a series, a polysiliconmemory effect material, a metal oxide or switchable complex metal oxidematerial, a carbon nanotube material, a graphene switchable resistivitymaterial, a phase change material, a conductive bridge element, anelectrolyte switching material, a switchable polymer material, or carbonresistivity switching material, such as amorphous, polycrystalline ormicrocrystalline carbon or graphite material.

The temporary resistor 120 can be any suitable resistor. For example theresistor can be a standard wire-wound resistor, a surface mountedresistor, or integrated into the same die as the memory cell 110. Thetemporary resistor 120 can be any value; however, the temporary resistor120 should be selected to provide current limiting protection for thediode 130. Additionally, multiple resistors can be used to create aneffective resistance.

The temporary resistor 120, the voltage source 160 and the switch 150can be part of a driving circuitry of the memory device. The drivingcircuitry includes circuitry for reading and writing the memory cells ofthe memory cell array. The temporary resistor 120 is used to program(write), but not read, the memory cell 110. The driving circuitry alsoincludes addressing logic used to individually address the individualmemory cells of the memory cell array. Hence, one temporary resistor canbe used to program the entire memory cell array. The switch 150 alsoallows the temporary resistor 120 to be disconnected. For example, thetemporary resistor 120 can be disconnected from the memory cell 110while the memory cell 110 is being read or reset by the other drivingcircuitry.

Referring to FIG. 2, a schematic of a memory cell 110 with multipletemporary resistors in accordance with a representative embodiment isshown. The memory cell 110 includes a diode and a resistivity switchingmaterial element. The memory cell 110 is part of a memory cell array ina memory device. During programming, a first temporary resistor 123, asecond temporary resistor 125, or a third temporary resistor 127 and avoltage source 160 are electrically connected to the memory cell 110 bya switch 150. In one example, the first temporary resistor 123 is a 100kΩ resistor, the second temporary resistor 125 is a 400 kΩ resistor, andthe third temporary resistor 127 is a 800 kΩ resistor. The memory cell110 and the voltage source 160 are grounded together.

Referring to FIG. 3, a top view of a memory cell 110 located on the samechip as a temporary resistor 120 in accordance with a representativeembodiment is shown. The memory cell 110 is part of an array of memorycells 315. The temporary resistor 120 and the array of memory cells 315can be integrated on a microchip 380. Where a temporary resistor and amemory cell are included in the same device, a switch or transistor isincluded to electrically isolate the resistor so that the memory cellcan be read, reset, or programmed using other techniques. Alternatively,temporary resistor 120 and the array of memory cells 315 can be onseparate chips but integrated into a single package.

Referring to FIG. 4, a top view of a memory cell 110 located on the samecircuit board as a temporary resistor 120 in accordance with arepresentative embodiment is shown. The memory cell 110 is part of anarray of memory cells 415 that are located on a memory device 480. Thememory device 480 is an encapsulated semiconductor package. Thetemporary resistor 120 and the memory device 480 can be integrated on aprinted circuit board 490. Where a temporary resistor and a memory cellare included in the same circuit board, a switch or transistor isincluded to electrically isolate the resistor so that the memory cellcan be read, reset, or programmed using other techniques. The switch andother driving logic can be located in the memory device 480 or as aseparate device located on the printed circuit board 490.

Referring to FIG. 5, a top view of a memory cell 110 separate from atemporary resistor 120 in accordance with a representative embodiment isshown. The memory cell 110 is part of an array of memory cells 515 thatare located on a memory device 580. The temporary resistor 120 can beremovable. For example, at a processing plant the temporary resistor 120can be temporarily attached to the memory device 580. After programming,the temporary resistor 120 is removed. The memory device 580 can then beassembled into other products or shipped to another plant for furtherassembly. Likewise, the voltage source can be part of an integrateddevice including memory cells or the voltage source can be removable.

Referring to FIG. 6, a graph of setting a memory cell with a 800 kΩresistor in accordance with a representative embodiment is shown. Duringprogramming, a reverse bias can be applied to a memory cell by a voltagesource using a temporary resistor in series with the memory cell. Inthis example, the temporary resistor is a 800 kΩ resistor. The memorycell includes a diode and a resistivity switching material element. Inthis example, the diode is a p-i-n diode fabricated using a 0.5 μmprocess, and a resistivity switching material element made of HfO₂.

A high reverse voltage (Vdiode+Vset) is applied on the memory cell asdepicted by I-V (current-voltage) curve 610. When the reverse biasvoltage reaches a switching point 620, the state of the resistivityswitching material element changes from a first initial state to asecond state different from the first state. The switching point is, forexample, about −15 volts to −25 volts across the temporary resistor, thediode, and the resistivity switching material element. The switchingpoint approximately occurs at a reverse bias threshold voltage thatdepends on diode reverse leakage characteristics of the diode and athreshold voltage required to set the resistivity switching materialelement, which vary based on the type of diode, and the type ofresistivity switching material element. The change is substantial;however, the change can be gradual.

Once the Vset is high enough to set the memory cell, the voltage onmemory cell drops quickly and the diode changes into its breakdowntransition region. After the resistivity switching material elementchanges state, the temporary resistor protects the diode in the memorycell by limiting the amount of current that can pass through the diode.Hence, the temporary resistor prevents diode breakdown from occurring.The diode remains operational, but the resistivity switching materialelement has changed state. From any state, the memory cell can bereturned, substantially, to its initial (original) state by applying aforward bias to the memory cell without the temporary resistor. Asubstantially initial state is any state within about the bounds of thedefinition of the initial state as defined by a reading circuit.

After the memory cell has been changed to the second state, a reversebias can be applied to the memory cell by the voltage source using asecond temporary resistor in series with the memory cell. Referring toFIG. 7, a graph of setting a memory cell with a 400 kΩ resistor inaccordance with a representative embodiment is shown. In this example,the temporary resistor is a 400 kΩ resistor. In this example, the memorycell has already been programmed with a 800 kΩ as in FIG. 6.

A high reverse voltage (Vdiode+Vset) is applied on the memory cell asdepicted by I-V (current-voltage) curve 710. When the reverse biasvoltage reaches a switching point 720, the state of the resistivityswitching material element changes from the second state to a thirdstate different from the second state or the first state. The switchingpoint is, for example, about −15 volts to −25 volts across the temporaryresistor, the diode, and the resistivity switching material element. Theswitching point approximately occurs at a reverse bias threshold voltagethat depends on diode reverse leakage characteristics of the diode and athreshold voltage required to set the resistivity switching materialelement, which vary based on the type of diode, and the type ofresistivity switching material element. The change is substantial;however, the change can be gradual.

Once the Vset is high enough to set the memory cell, the voltage onmemory cell drops quickly and the diode changes into its breakdowntransition region. After the resistivity switching material elementchanges state, the temporary resistor protects the diode in the memorycell by limiting the amount of current that can pass through the diode.Hence, the temporary resistor prevents diode breakdown from occurring.The diode remains operational, but the resistivity switching materialelement has changed state. From any state, the memory cell can bereturned, substantially, to its initial (original) state by applying aforward bias to the memory cell without the temporary resistor.

After the memory cell has been changed to the third state, a reversebias can be applied to the memory cell by the voltage source using athird temporary resistor in series with the memory cell. Referring toFIG. 8, a graph of setting a memory cell with a 200 kΩ resistor inaccordance with a representative embodiment is shown. In this example,the temporary resistor is a 200 kΩ resistor. In this example, the memorycell has already been programmed with a 400 kΩ as in FIG. 7.

A high reverse voltage (Vdiode+Vset) is applied on the memory cell asdepicted by I-V (current-voltage) curve 810. When the reverse biasvoltage reaches a switching point 820, the state of the resistivityswitching material element changes from the third state to a fourthstate different from the third state, the second state or the firststate. The switching point is, for example, about −15 volts to −25 voltsacross the temporary resistor, the diode, and the resistivity switchingmaterial element. The switching point approximately occurs at a reversebias threshold voltage that depends on diode reverse leakagecharacteristics of the diode and a threshold voltage required to set theresistivity switching material element, which vary based on the type ofdiode, and the type of resistivity switching material element. Thechange is substantial; however, the change can be gradual.

Once the Vset is high enough to set the memory cell, the voltage onmemory cell drops quickly and the diode changes into its breakdowntransition region. After the resistivity switching material elementchanges state, the temporary resistor protects the diode in the memorycell by limiting the amount of current that can pass through the diode.Hence, the temporary resistor prevents diode breakdown from occurring.The diode remains operational, but the resistivity switching materialelement has changed state. From any state, the memory cell can bereturned, substantially, to its initial (original) state by applying aforward bias to the memory cell without the temporary resistor.

After the memory cell has been changed to the fourth state, a reversebias can be applied to the memory cell by the voltage source using afourth temporary resistor in series with the memory cell. Referring toFIG. 9, a graph of setting a memory cell with a 100 kΩ resistor inaccordance with a representative embodiment is shown. In this example,the temporary resistor is a 100 kΩ resistor. In this example, the memorycell has already been programmed with a 200 kΩ as in FIG. 8.

A high reverse voltage (Vdiode+Vset) is applied on the memory cell asdepicted by I-V (current-voltage) curve 910. When the reverse biasvoltage reaches a switching point 920, the state of the resistivityswitching material element changes from the fourth state to a fifthstate different from the fourth state, the third state, the second stateor the first state. The switching point is, for example, about −15 voltsto −25 volts across the temporary resistor, the diode, and theresistivity switching material element. The switching pointapproximately occurs at a reverse bias threshold voltage that depends ondiode reverse leakage characteristics of the diode and a thresholdvoltage required to set the resistivity switching material element,which vary based on the type of diode, and the type of resistivityswitching material element. The change is substantial; however, thechange can be gradual.

Once the Vset is high enough to set the memory cell, the voltage onmemory cell drops quickly and the diode changes into its breakdowntransition region. After the resistivity switching material elementchanges state, the temporary resistor protects the diode in the memorycell by limiting the amount of current that can pass through the diode.Hence, the temporary resistor prevents diode breakdown from occurring.The diode remains operational, but the resistivity switching materialelement has changed state. From any state, the memory cell can bereturned, substantially, to its initial (original) state by applying aforward bias to the memory cell without the temporary resistor.

The memory cell can be read by applying a forward bias to the diode.Referring to FIG. 10, a graph comparing reading a memory cell byapplying a forward bias in multiple states in accordance with arepresentative embodiment is shown. During reading, a forward bias canbe applied to a memory cell by a voltage source without using atemporary resistor in series with the memory cell. As above, the diodeis a p-i-n diode fabricated using a 0.5 μm process, and a resistivityswitching material element made of HfO₂.

Because different temporary resistors limit the reverse current todifferent levels, the memory cell is set to different states.Additionally, the memory cell has distinct states before and afterprogramming. Each state has a different I-V (current-voltage) profile.

FIG. 10 shows I-V profiles of a memory cell read after being programmedwith various temporary resistors. The first I-V profile 1010 shows thecurrent readings when an un-programmed memory cell is forward biasedfrom 0V to 2V. For example, the current reading at a forward bias of 2Vis about 2.00E-11 A. The second I-V profile 1020 shows the currentreadings when a memory cell programmed with a 800 kΩ resistor is forwardbiased from 0V to 2V. For example, the current reading at a forward biasof 2V is about 1.00E-7 A. The third I-V profile 1030 shows the currentreadings when a memory cell programmed with a 400 kΩ resistor is forwardbiased from 0V to 2V. For example, the current reading at a forward biasof 2V is about 2.00E-5 A. The fourth I-V profile 1040 shows the currentreadings when a memory cell programmed with a 200 kΩ resistor is forwardbiased from 0V to 2V. For example, the current reading at a forward biasof 2V is about 1.50E-5 A. The fifth I-V profile 1050 shows the currentreadings when a memory cell programmed with a 100 kΩ resistor is forwardbiased from 0V to 2V. For example, the current reading at a forward biasof 2V is about 1.00E-4 A. Hence, for at least in the 0.75V to 2V range,the memory cells programmed with distinct resistors produce distinctcurrent readings.

Advantageously, the different states of a resistivity switching materialelement can represent different data values in a rewritable or one-timeprogrammable memory. For example, an initial state can represent datavalue ‘00’; a state created with a 800 kΩ resistor can represent datavalue ‘01’; a state created with a 400 kΩ resistor can represent datavalue ‘10’; and a state created with a 100 kΩ resistor can representdata value ‘11’. Hence, any number of states and, therefore, data valuescan be stored in the resistivity switching material elements based uponthe number of resistors available and the sensitivity of the readingcircuit. Resistors can be combined in series and parallel to increasethe number of resistance values available while reducing the number ofresistors needed.

After programming, the diode in the memory cell remains operational.Referring to FIG. 11, a graph depicting an I-V profile 1110 of a diodein a memory cell after being programmed with a 100 kΩ resistor inaccordance with a representative embodiment is shown. As above, thediode is a p-i-n diode fabricated using a 0.5 μm process, and aresistivity switching material element made of HfO2. In this example,the memory cell was programmed using a 100 kΩ resistor. The I-V profile1110 was created by forward biasing the diode from the memory cell from−8V to 2V without the 100 kΩ resistor. I-V profile 1110 shows the I-Vcurve of a working diode; specifically, the curve show the traditionalswitching effect at 1V. Advantageously, programming the memory cellusing a reverse bias with temporary resistor technique does not damagethe diode.

Advantageously, fresh memory cells produce repeatable results. Referringto FIG. 12, a graph depicting an I-V profile of setting a fresh memorycell with a 800 kΩ resistor in accordance with a representativeembodiment is shown. In this example, a reverse bias is applied to afresh (un-programmed) memory cell by a voltage source using a 800 kΩtemporary resistor in series with the memory cell. The memory cellincludes a p-i-n diode fabricated using a 0.5 μm process, and aresistivity switching material element made of HfO₂.

A high reverse voltage (Vdiode+Vset) is applied on the memory cell asdepicted by I-V (current-voltage) curve 1210. When the reverse biasvoltage reaches a switching point 1220, the state of the resistivityswitching material element changes from a first initial state to asecond state different from the first state. The switching point isabout −15 volts to −25 volts across the temporary resistor, the diode,and the resistivity switching material element.

Once the Vset is high enough to set the memory cell, the voltage onmemory cell drops quickly and the diode changes into its breakdowntransition region. After the resistivity switching material elementchanges state, the temporary resistor protects the diode in the memorycell by limiting the amount of current that can pass through the diode.Hence, the temporary resistor prevents diode breakdown from occurring.

Referring to FIG. 13, a graph depicting an I-V profile of setting asecond fresh memory cell with a 100 kΩ resistor in accordance with arepresentative embodiment is shown. In this example, a reverse bias isapplied to a fresh (un-programmed) memory cell by a voltage source usinga 100 kΩ temporary resistor in series with the memory cell. The memorycell includes a p-i-n diode fabricated using a 0.5 μm process, and aresistivity switching material element made of HfO₂.

A high reverse voltage (Vdiode+Vset) is applied on the memory cell asdepicted by I-V (current-voltage) curve 1310. When the reverse biasvoltage reaches a switching point 1320, the state of the resistivityswitching material element changes from a first initial state to asecond state different from the first state. The switching point isabout −15 volts to −25 volts across the temporary resistor, the diode,and the resistivity switching material element.

Once the Vset is high enough to set the memory cell, the voltage onmemory cell drops quickly and the diode changes into its breakdowntransition region. After the resistivity switching material elementchanges state, the temporary resistor protects the diode in the memorycell by limiting the amount of current that can pass through the diode.Hence, the temporary resistor prevents diode breakdown from occurring.

Referring to FIG. 14, a graph comparing readings from a first freshmemory cell programmed with a 800 kΩ resistor and a second fresh memorycell programmed with a 100 kΩ resistor in accordance with arepresentative embodiment is shown. The first I-V profile 1410 shows thecurrent readings when the first fresh memory cell, programmed with the100 kΩ resistor, is forward biased from 0V to 2V. For example, thecurrent reading at a forward bias of 2V is about 1.00E-4 A. The secondI-V profile 1420 shows the current readings when the second fresh memorycell, programmed with the 800 kΩ resistor, is forward biased from 0V to2V. For example, the current reading at a forward bias of 2V is about1.00E-7 A. Hence, for at least in the 0.75V to 2V range, the memorycells programmed with distinct resistors produce distinct currentreadings. Additionally, the first I-V profile 1410 of the first freshmemory cell, programmed with the 100 kΩ resistor, matches the I-Vprofile of the memory cell programmed with a 100 kΩ resistor of FIG. 10(the memory cell of FIG. 10 was programmed multiple times). Likewise,the second I-V profile 1420 of the second fresh memory cell, programmedwith the 800 kΩ resistor, matches the I-V profile of the memory cellprogrammed with a 800 kΩ resistor of FIG. 10 (the memory cell of FIG. 10was programmed multiple times).

Reverse Bias of a Memory Cell with a Carbon Storage Element

In another embodiment, the external resistor 120 shown in FIG. 1 of theprior embodiment can be omitted, if desired. In this embodiment, thestorage element comprises a carbon storage element, and the reliabilityof setting and resetting the carbon storage element can be improved. Ina typical forward bias situation, a carbon storage element, such as acarbon nanotube material, has long SET times and high RESET voltages.The SET time for a carbon storage element is in the ranges ofmilliseconds. High RESET voltages are used because the SET voltages arehigh and variable. The RESET voltages are usually higher than the SETvoltages.

A carbon storage element is SET when a resistivity state of the carbonstorage element is changed from a higher resistivity to a lowerresistivity. A carbon storage element is RESET when a resistivity stateof the carbon storage element is changed from a lower resistivity to ahigher resistivity. A carbon storage element can be a resistivityswitching material selected from a carbon nanotube material, anamorphous or a polycrystalline carbon resistivity switching material, agraphene switchable resistance material, or a combination thereof.Without wishing to be bound by a particular theory, the inventorsbelieve that when a SET voltage is applied across a carbon storageelement, linkages are formed between portions of a carbon material ofthe carbon storage element. For example, where the carbon storageelement includes carbon nanotubes, linkages or filaments are formedbetween at least some of the carbon nanotubes when a SET voltage isapplied thereby increasing the conductivity of the carbon storageelement. When a RESET voltage is applied, the RESET voltage/currentcauses the linkages or filaments between the carbon nanotubes to bedegraded or destroyed thereby decreasing the conductivity of the carbonstorage element. Thus, the RESET voltage/current should be higher thanthe SET voltage/current in order to degrade or destroy the linkages.When the SET current is high and/or variable, the RESET current must bethat much higher in order to maintain consistent performance. The highervoltages and currents result in wasted power and shorter lifetimes in amemory array. A carbon storage element is typically formed in the SETstate. Before programming an array of memory cells for the first time,it is common to RESET the carbon storage elements.

Referring to FIG. 15, a schematic of a carbon storage element testcircuit in accordance with a representative embodiment is shown. Thetest circuit includes a carbon storage element 141 in series with aresistor 1540, such as a 23.8 kiloOhm resistor. The carbon storageelement 141 can be part of a memory cell array in a memory device. Thetest circuit includes test points 1510, 1520, and 1530. The carbonstorage element 141 is initially in a RESET state (high resistance).

Referring to FIG. 16, a graph of a forward bias of the carbon storageelement test circuit of FIG. 15 in accordance with a representativeembodiment is shown. The graph shows the voltage (V) versus time(seconds) for various test points of the carbon storage element testcircuit of FIG. 15 in a forward biased state. Plot 1610 shows thevoltage of a forward biasing pulse applied across test points 1510 and1530. The forward biasing pulse lasts for approximately 5 microseconds.Plot 1620 shows the voltage measured across test points 1520 and 1530,or the voltage across the resistor 1540. Plot 1630 shows the groundvoltage relative to the test equipment (not shown).

Plot 1620 shows that the voltage across the resistor 1540 jumps orchatters between about 2 and 3 V while the forward biasing pulse isapplied. The chattering is caused by the storage element 141 setting andresetting because too much current is available after the storageelement 141 sets. For example, the carbon storage element 141 isinitially in a RESET state (high resistance). When the carbon storageelement 141 sets (low resistance) the carbon storage element 141continues to conduct and the available current from the test circuit isstill capable of resetting the carbon storage element 141. As describedabove, the inventors believe that the chattering is caused by theformation and destruction of filaments or linkages in the carbonmaterial during the SET and RESET operations.

In previous approaches, a CMOS control element is used during the SETand the RESET operations. After a SET operation, the CMOS control cancontinue to provide high current to a storage element due to surge andthe capacitance of the MOS device. Hence, the chatter is not completelyeliminated.

Referring to FIG. 17, a graph of read current variability for tenforward biased carbon storage element memory cells in accordance with arepresentative embodiment is shown. The graph shows the read current(Read current @2V) (A) versus set/reset cycles (Cyc#) for the ten carbonstorage element memory cells where the carbon storage element is SET andRESET by forward biasing the diode of the memory cell. Plots 1710 arethe read current measurements for memory cells in the SET state (Ion).Plots 1720 are the read current measurements for memory cells in theRESET state (Ioff). Plots 1710 and 1720 show that the variation in theSET state can cause variation in the RESET state. Thus, improved methodsof and structures for controlling the SET current after SET are needed.

Referring to FIG. 18, a current-voltage (I-V) graph of a diode of amemory cell in accordance with a representative embodiment is shown. Thegraph shows the absolute current (A) versus voltage (V) for a diode of amemory cell in a memory array. Plot 1810 shows the current of the diodein reverse bias and in forward bias. Area 1840 is the breakdown regionof the diode in reverse bias. When the diode is forward biased, such asat point 1820, there is much more current available then when the diodeis reverse biased, such as at point 1830. Notably, the reverse bias areaof plot 1810 shows that reverse bias current is very flat compared tothe forward bias current. Thus, the current in reverse bias can becontrolled with much more precision.

Referring to FIG. 19, a perspective view of a memory cell 1910 inaccordance with a representative embodiment is shown. The memory cell1910 includes a diode steering element 1930 and a carbon storage element1920 electrically connected in series. The diode steering element 1930and the carbon storage element 1920 can be pillar shaped. The diodesteering element 1930 can be located over or under the storage element1920. The memory cell 1910 can be located in a monolithic threedimensional array of nonvolatile memory cells.

The carbon storage element 1920 can be electrically connected to a bitline 1940. The diode steering element 1930 can be electrically connectedto a word line 1950. The bit line 1940 and the word line 1950 can eachbe can be any one or more metals or conductors, for example, aluminum,copper, titanium nitride, tungsten, titanium, titanium silicide, etc.The carbon storage element 1920 and the bit line 1940 can be connectedby a contact 1992. The diode steering element 1930 and the carbonstorage element 1920 can be connected by a contact 1996. The diodesteering element 1930 and the word line 1950 can be connected by acontact 1998. The contacts can be any one or more metals or conductors,for example, titanium nitride, tungsten, titanium, titanium silicide,etc. The diode steering element 1930 and the carbon storage element 1920can be a nonvolatile memory cell located over a lower electrode, such asword line 1950 and under an upper electrode, such as bit line 1940.Alternatively, line 1950 may be a bit line and line 1940 may be a wordline.

The diode steering element 1930 can be a p-n semiconductor diode, ap-i-n semiconductor diode, or a punch-through diode. Alternatively, thediode steering element 1930 can be a metal insulator metal (MIM) diode,a metal insulator-insulator metal (MIIM) diode, a tunneling diode, orany two terminal non-linear conducting device. The diode steeringelement 1930 can be made of polysilicon, single crystal silicon,amorphous silicon or a combination thereof. Other semiconductormaterials, such as Ge, SiGe, III-V materials, etc. may also be used.

The carbon storage element 1920 can be a resistivity switching materialselected from a carbon nanotube material, an amorphous or apolycrystalline carbon resistivity switching material, a grapheneswitchable resistance material, or a combination thereof.

Referring to FIG. 20, a flowchart of programming the memory cell of FIG.19 in accordance with a representative embodiment is shown. Additional,fewer, or different operations may be performed depending on theparticular embodiment. The operations may also be performed in adifferent order than that shown and described. In an operation 2010, thememory cell 1910 can be provided. As discussed above, the memory cell1910 includes a diode steering element 1930 and a carbon storage element1920.

In an operation 2020, a first voltage can be provided to the memorycell, such that the diode steering element 1930 is reverse biased andthe carbon storage element 1920 is SET to a lower resistivity state. Thefirst voltage can be any magnitude, for example, in the range of 2 V to10 V. The first voltage can be provided by a driving circuit, acapacitive discharge circuit, a capacitive discharge of a chargetemporarily stored on the bit line 1940 and the word line 1950, or avoltage pulse.

The capacitive discharge circuit (described further below) and pulsemethod are disclosed in U.S. patent application Ser. No. 12/339,338,filed Dec. 19, 2008, entitled “Capacitive Discharge Method for Writingto an NVM” which has been incorporated by reference. The first voltagecauses a first current to SET the carbon storage element 1920.

In an operation 2030, a second voltage can be provided to the memorycell, such that the diode steering element 1930 is forward biased andthe carbon storage element 1920 is RESET to a higher resistivity state.The second voltage causes a second current to RESET the carbon storageelement 1920. A magnitude of the second current associated with thesecond voltage is greater than a magnitude of the first currentassociated with the first voltage as shown in FIG. 18. The carbonstorage element 1920 can be SET and RESET a plurality of times byapplying a reverse bias and forward bias, respectively.

Referring to FIG. 21, a graph of the read current and programmingvoltage for multiple programming cycles of the memory cell of FIG. 19 inaccordance with a representative embodiment is shown. FIG. 21 shows datawhen the SET operation is done in reverse mode and the RESET operationis done in forward mode. On the left scale, the graph shows the voltage(V) used to SET and RESET versus cycle (#) for the memory cell of FIG.19. Plot 2110 shows the SET voltage used to reverse bias the diodesteering element 1930 of the memory cell of FIG. 19. Plot 2140 shows theRESET voltage used to forward bias the diode steering element 1930 ofthe memory cell of FIG. 19. On the right scale, the graph shows the readcurrent (Current (A) @2V) versus cycle (#) for the memory cell of FIG.19. Plot 2120 shows the read current taken after the memory cell is SETand forward biased at a read voltage of 2V (Ion@2V). Plot 2130 shows theread current taken after the memory cell is RESET and forward biased ata read voltage of 2V (Ioff@2V). Point 2150 represents the read currentof the diode in its virgin state.

The SET times are about 5 microseconds to 50 microseconds and the “on”state (a read current after a set) is controlled between 1.5-4microamps. The setting voltage is 8V in reverse and there is no diodedegradation of the diode steering element 1930.

At the beginning of a SET with reverse bias, most of the voltage istaken up by the carbon storage element 1920 (as the material is in thehigh resistance state). As the carbon storage element 1920 starts to SET(lower resistance), the voltage then gets distributed between the diodesteering element 1930 and the carbon storage element 1920. The SEToperation stops at a quiescent point. The current provided in reversebias cannot be higher than the reverse current (the breakdown current)of the diode steering element 1930 which is limited by the diode's shareof the distributed set voltage. Higher reverse currents require highervoltages on the diode and therefore limit the switching element voltageto about one or two volts. Hence simultaneous high reverse current andvoltage on the switching element is avoided and the carbon storageelement 1920 can avoid a RESET immediately after a SET (i.e., it isbelieved that the newly formed linkages or filaments formed in thecarbon material are not broken or destroyed during the SET operation).Advantageously, the steady current during a reverse bias SET eliminateschatter and reduces the SET time. In addition, the steady current duringa reverse bias SET results in consistent read currents of memory cellsin the SET state. In contrast, the during RESET operation about one ortwo volts of forward bias across the diode allow about 10 micro Amps ormore to flow thru the cell. The rest (about 3 or 4 Volts) of the resetvoltage is applied to the switching element. This larger forward biasapplied to the switching element, and higher current through the carbonmaterial is believed to break or destroy the linkages or filaments inthe carbon material to increase the resistivity of the carbon material.

Reverse Bias by Capacitive Discharge of the Bit and Word Lines

Referring to FIG. 22, a circuit diagram of a memory cell array inaccordance with a representative embodiment is shown. The memory cellarray 2200 includes a word line (i.e., row) decoder 2210, a bit line(i.e., column) decoder 2230, word lines 2212 and 2214, bit lines 2231and 2233, and memory cells 2240, 2245, 2250, and 2255. Each of thememory cells 2240, 2245, 2250, and 2255 is connected to one bit line(for instance, memory cell 2255 is connected to bit line 2233) and oneword line (for instance, memory cell 2255 is connected to word line2212). Each of the memory cells 2240, 2245, 2250, and 2255 include adiode steering element 1930 and a carbon storage element 1920.

The word line decoder 2210 controls dual device drivers 2213 and 2215.Alternatively, other word line device drivers can be used. Each of thedual device drivers 2213 and 2215 drives one of the word lines 2212 and2214, respectively. Each of the dual device drivers 2213 and 2215includes a PMOS transistor 2262 and a NMOS transistor 2261. In thisexample, the NMOS transistor 2261 is connected (e.g., the source regionof the NMOS is connected) to a word line unselect voltage source 2270(V(WL VSEL)). The word line unselect voltage source 2270 provides a wordline unselect voltage, such as 0V or ground. The word line unselectvoltage source can change based on the operation performed on a memorycell. The PMOS transistor 2262 is connected (e.g., the source region ofthe PMOS is connected) to a word line select voltage source 2260 (V(WLSEL)). During a SET operation, the word line select voltage source 2260provides a voltage of half of the SET voltage, positively biased (+½Vset). The SET voltage can be the voltage needed to lower theresistivity of a carbon storage element, as described above. The wordline select voltage source can change based on the operation performedon a memory cell, for instance, during a read, the word line selectvoltage source can be ground.

Various relative voltages can be chosen for the word line select andunselect voltage sources to control the amount of leakage through thememory cells. Generally, the biasing voltages can be adjusted to accountfor the turn-on voltages of the memory cells. In FIG. 22, when the wordline decoder 2210 applies an unselect control voltage to the dual devicedrivers (2213, 2215), the NMOS transistor 2261 is activated and applies(e.g., via the NMOS drain) the word line unselect voltage to the wordlines (2212, 2214); the PMOS transistor 2262 is deactivated. Conversely,the when the word line decoder 2210 applies a select control voltage tothe dual device drivers (2213, 2215), the PMOS transistor 2262 isactivated and applies the word line select voltage to the word lines(2212, 2214) via its drain; the NMOS transistor 2261 is deactivated.

The bit line decoder 2230 controls single device drivers 2237 and 2239.Each of the single device drivers 2237 and 2239 drives one of the bitlines 2233 and 2231, respectively. Each of the single device drivers2237 and 2239 includes a MOSFET transistor (2234, 2232). The MOSFETtransistor (2232, 2234) can be a NMOS or PMOS transistor. The MOSFETtransistors (2232, 2234) can each be isolated using a triple well 2236.For example, by placing the NMOS transistor in a p-well (where thep-well is in an n-well that is in a p-substrate), negative voltages canbe used. The use of negative voltages allows all unselect bias voltagesto be ground voltage in the reverse bias set operation, and manyvoltages on chip can be reduced by ½ VPP (where VPP is the maximumvoltage applied to the memory cells). The use of negative voltages alsoallows a reduction in the maximum stress applied to any MOSFET driverdevice to about ½ VPP. This arrangement saves power and causes lessstress on the circuit. The triple well 2236 is biased by a well voltage2238. The well voltage 2238 is the most negative bias for the NMOSdriver circuit.

Each of the single device drivers 2237 and 2239 can be connected to adriving circuit 2284 by data bus 2280 (SELB). The driving circuit 2284can be connected to a bit line select voltage source 2295 (V(BL SEL))and a bit line unselect voltage source 2290 (“V(BL UNSEL)”). The bitline select voltage source 2295 is equal to about of half of the SETvoltage, negatively biased (−½ Vset). The bit line unselect voltagesource 2290 provides a bit line unselect voltage, such as 0V or ground.The bit line select voltage source can change based on the operationperformed on a memory cell, for instance, during a read, the bit lineselect voltage source can be VPP. Various relative voltages can bechosen for the bit line select and unselect voltage sources to controlthe amount of leakage through the memory cells. Generally, the biasingvoltages can be adjusted to account for the turn-on voltages of thememory cells.

Alternatively, the orientation of the memory cells 2240, 2245, 2250, and2255 can be reversed so their “anodes” and “cathodes” are interchanged.The PMOS transistors are replaced with NMOS transistors and vice-versa.Hence, the select and unselect voltages are reversed in polarity.Alternatively, the single device drivers and the dual device drivers canoptionally include pull-up or pull-down resistors. In other embodimentsthe bit line driver is implemented by a two device driver including oneNMOS device and one PMOS device.

Referring to FIG. 23, a flowchart of programming the memory cell array2200 of FIG. 22 in accordance with a representative embodiment is shown.Additional, fewer, or different operations may be performed depending onthe particular embodiment. The operations may also be performed in adifferent order than that shown and described. In an operation 2310, amemory cell array including a plurality of X lines (e.g., word lines), aplurality of Y lines (e.g., bit lines), and a plurality of memory cellsis provided. Each memory cell includes a diode steering element inseries with a carbon storage element and each memory cell can beelectrically connected between one of the plurality of X lines and oneof the plurality of Y lines. For example, the memory cell array can bethe memory cell array 2200 of FIG. 22. In one example the charging bitline bias is minus one half the Vset, and the word line bias is plushalf the Vset. The anode of the memory cell is connected to the bit lineand the cathode to the word line.

In an operation 2320, a selected X line of the plurality of X linesassociated with at least one selected memory cell of the plurality ofmemory cells is driven to a X line select voltage. For example, selectedword line 2212 is driven to the word line select voltage, +½ Vset. Thepulse duration may be hundreds of nano seconds to hundreds of microseconds so the word line bias is applied before during and after the bitline pulse is applied. The remaining unselected word lines 2214 aredriven to the word line unselect voltage, for example, ground. Inanother embodiment, the word line pulse is short (about 10 to 100 ns)and the bit line pulse is longer.

In an operation 2330, at least one selected Y line of the plurality of Ylines associated with the at least one selected memory cell 2255 isdriven to a Y line select voltage. The pulse duration may be 10 to 100ns. The X line select voltage and the Y line select voltage areconfigured such that a diode steering element of the at least oneselected memory cell is reverse biased. For example, the selected bitline 2233 is driven to the bit line select voltage, −½ Vset. Theremaining unselected bit lines 2231 are driven to the bit line unselectvoltage, for example, ground. While bit line 2233 and word line 2212 arebeing driven, a charge is building across bit line 2233 and word line2212. Alternatively, many bit lines can be selected at the same time.

Thus, memory cell 2255 is now selected, indicated by ‘S’. The diode ofMemory cell 2255 is reverse biased at the setting voltage. Memory cells2250 are now half-selected, indicated by ‘H’. The diodes of memory cells2250 are slightly reversed biased, but not enough to conduct asignificant amount of current. Memory cells 2245 are now half-selected,indicated by ‘H’. The diodes of memory cells 2245 are slightly reversebiased, but not enough to conduct a significant amount of current.Therefore, the half selected memory cells (2250, 2245) will not conductenough current to SET or reset. Memory cells 2240 are now unselectedindicated by ‘U’. Memory cells 2240 are not biased and will not conductcurrent.

In an operation 2340, the at least one selected Y line of the pluralityof Y lines is isolated before a carbon storage element of the at leastone selected memory cell is completely SET to a lower resistivity state(i.e., before reaching a predetermined resistivity state when all of thelinkages or filaments in the carbon material needed to reach thispredetermined resistivity state are formed). The isolation duration maybe 10 ns to several microseconds (e.g., 10 ns to 10 microseconds). Forexample, MOSFET transistor 2234 of the single device driver 2237, whichis connected to bit line 2233, can be turned off. The bit line 2233 cannow be floating relative to the driving circuit 2284. When the MOSFETtransistor 2234 is turned off, the charge that has built across bit line2233 and word line 2212 still remains on bit line 2233. However, thecarbon storage element 1920 of memory cell 2255 has not completely SETyet. That is, the carbon storage element 1920 of memory cell 2255 hasnot reached the lower resistivity state associated with the SET state.Notably, the plate area of a bit line is large relative to thecross-section of a diode steering element.

In an operation 2350, the at least one selected Y line is held in anisolated state until the carbon storage element of the at least oneselected memory cell is completely SET to the lower resistivity state.For example, the charge that has built across bit line 2233 and wordline 2212 can still provide current to the carbon storage element 1920of memory cell 2255. Eventually, the charge will cause the carbonstorage element 1920 to set, or completely change to a lower resistivitystate (e.g., when all of the desired linkages or filaments in the carbonmaterial are formed). After the storage element is completely SET, theisolation of the selected bit line can be stopped and the next operation(e.g., a read or programming operation) can be performed by applying abias to the bit line. Advantageously, the capacitive discharge of bitline 2233 produces a limited amount of current determined by bit lineand world line capacitance and the cell resistance which protects thediode from damage during the SET process. Advantageously, chatter or thepossibility of RESET after SET is greatly reduced because the voltagedecrease across the cell is synchronized with the SET of the cell. Itshould be noted that an isolated bit line includes the case where anegligible (up to 100 nA) current flows to the bit line during the SEToperation. For some large arrays it is possible that tens of nano ampsof current may flow to the bit line, regardless of whether it isintended or not. Even deliberate 50 to 100 nAmp currents would not avoidthe above described capacitive set operation.

In the case where more than one bit line is selected and charged, theselected bit lines are all isolated quickly, but the selected cells maySET at various time (e.g., by design or due to variability in the carbonmaterial). As selected cells become set, each associated bit linecharges to reduce the voltage across the cell and remain set. Thevoltage drop in the word line decreases as more selected cells SET andmore bit lines discharge providing better word line voltage for theremaining or harder to SET bits. In alternative embodiments, the biasingvoltage between a bit line and a word line can have variousconfigurations including splitting the biasing voltage 100/0, 50/50,25/75, 75/25, 0/100, etc. where, for example 0 denotes ground (or 0 V)and 100 denotes the full magnitude of the biasing voltage.

Referring to FIG. 24, a graph of a bit line voltage in accordance with arepresentative embodiment is shown. The graph shows the voltage (V)versus time (sec) on a selected bit line, for example, selected bit line2233. Plot 2410 shows the voltage changes that occur on selected bitline 2233 during the operations of FIG. 23. In region 2420, bit line2233 is driven to the bit line select voltage, −½ Vset, thereby reversebiasing the diode steering element 1930 of memory cell 2255. In region2430, bit line 2233 is isolated or floated leaving a charge on bit line2233. In region 2440, the carbon storage element 1920 of memory cell2255 sets, changing to a lower resistivity. The charge on bit line 2233is partially consumed by the state change of the carbon storage element1920. The remaining charge on bit line 2233 can be drained to the wordline 2212 as the carbon storage element 1920 becomes more conductive.The carbon storage element 1920 sets during the wait time and the biasacross the cell drops. In typical bias cases, the bit line voltage rises(absolutely) closer to the word line bias.

Advantageously, since the current is bounded by the charge availablefrom the isolated bit line, the carbon storage element 1920 can avoid aRESET immediately after a SET by limiting surge current. Advantageously,the steady current during a reverse bias SET reduces or eliminateschatter and reduces the SET time. In addition, the steady current duringa reverse bias SET results in consistent read currents of memory cellsin the SET state.

Reverse Bias by Capacitive Discharge Circuit

Rather than isolating just the bit line, the data bus connected to thebit line can be isolated from driving circuitry but still connected tothe selected cell so a large capacitance is discharged through the cellin the SET process. Various circuits for controlling the capacitivedischarge can be used. For example, the capacitive discharge circuitsdisclosed in U.S. patent application Ser. No. 12/339,338, filed Dec. 19,2008, entitled “Capacitive Discharge Method for Writing to an NVM” whichhas been incorporated by reference, can be used. These circuits can bemodified for reverse biasing by forming the electronic dual of thecircuitry, i.e. where all NMOS devices are changed to PMOS and biaseschange polarity. Alternatively the cell could be inverted such that theanode is connected to the word line and the cathode is connected to thebit line. The biasing voltage between a bit line and a word line canhave various configurations including splitting the biasing voltage100/0, 50/50, 25/75, 75/25, 0/100, etc. where, for example 0 denotesground (or 0 V) and 100 denotes the full magnitude of the biasingvoltage.

Referring to FIG. 25, a schematic diagram of a capacitive dischargecircuit in accordance with a representative embodiment is shown. In someembodiments, there will be one such circuit for each bit line or a groupof such circuits that can be selectively connected to different groupsof bit lines.

In some embodiments, it is desirable to select the word line firstbecause in some monolithic three dimensional memory arrays the word lineselection is slow. The charge may be placed on the bit line capacitancevery quickly by charge sharing as illustrated in FIG. 25. An additionalcapacitor 25708 is charged to the highest voltage (relatively) availablein the circuitry during a pre-charge time. Then, the bit line isselected through driver device 25610 and a charge sharing device 25710is turned on to connect this capacitor to the Data Bus (SELB) and thebit line (BL). The connected capacitors (25708, 25712, 25704) quicklyreach a desired voltage for the SET operation determined by thecapacitance ratio, and then the charge sharing device is shut off. TheSET operation occurs after the bit line receives the charge transferbecause it takes a longer time to SET the reversibleresistance-switching element than to transfer the charge.

The circuit of FIG. 25 includes a memory cell 25702 that includes acarbon storage element and a diode, as described above with respect toFIG. 19. Memory cell 25702 is connected to a bit line BL having acapacitance 25704. In one embodiment, capacitance 25704 is 1 pf. The bitline BL is connected to a Data bus via the BL Selection Circuit. In oneembodiment, each bit line has its own BL driver devices (25610, 25612),many bit lines can share other portions of the selection circuitryrepresented by symbol 25614, and many bit lines can be connected to amulti-line Data Bus.

The Data bus is connected to the bit line via transistor 25610. The Databus, which comprises capacitance 25712 (e.g. 2 pf), is connected totransistor 25710 which controls the charge sharing. The gate oftransistor 25710 receives a pulse (XPG_PULSE). Between pulses, the Databus (node SELB) is floating and isolated from node GSELB. During thepulse (a positive pulse), the data bus (node SELB) is connected toGSELB. Capacitor 25708 (e.g. 2 pf) is connected to ground from GSELB.

Transistor 25706, which is connected to −VRR and to GSELB, receives apulse (XSA_ENABLE). Between pulses, GSELB is floating. During a positivepulse, −VRR is used to charge GSELB without a current limit. −VRR isslightly greater in magnitude than −½ Vset. VRR is the most negativevoltage for the bit line driver and the decoder circuitry. Whentransistor 25710 receives a pulse at its gate, the charge at GSELB isused to charge SELB and the bit line to (−VRR)×(Capacitance of Databus)/(Capacitance of Data bus+Capacitance of GSELB+Capacitance of BL).The charge from GLELB transferred to the bit line charges the bit lineto the desired −½ Vset.

The circuit of FIG. 25 also includes a comparator 25720 which comparesthe voltage at GSELB with a reference Vref. When the comparator sensesthe discharge of the data bus and bit line, it concludes that a SET hassuccessfully occurred and outputs a SET detection signal indicating thatthe memory cell has been SET. The output of comparator 25720 is providedto the control logic for the memory system.

Referring to FIG. 26, a timing diagram that explains various embodimentsfor operating the circuit of FIG. 25 in accordance with a representativeembodiment is shown. Between t1 and t2, a pulse is applied to transistor25706 by the signal XSA_ENABLE. This charges GSELB without a currentlimit, as depicted. Between t3 and t4, a pulse is applied to transistor25710 by the signal XPG_PULSE. This cause the charge to be shared withSELB. The BL Selection circuit allows that charge to be shared with thebit line, as depicted in FIG. 26. In some cases, this one iteration willcause the memory cell to be SET. In other embodiments, multipleiterations of the two pulses (charging GSELB and charge sharing) will beused to increase the charge on the bit line until the memory cell is SET(see t5).

Referring to FIG. 27, a schematic diagram of another embodiment of acircuit that can be used to SET a memory cell using the capacitivedischarge described above in accordance with a representative embodimentis shown. In some embodiments, there will be one such circuit for eachbit line or a group of such circuits that can be selectively connectedto different groups of bit lines. In the circuit of FIG. 27, the bitline selection device is turned off before the memory cell is switchedinto the new state.

The circuit of FIG. 27 includes a memory cell 27750 that includes acarbon storage element and a diode, as described above with respect toFIG. 19. Memory cell 27750 is connected to a bit line BL having acapacitance 27752. The bit line BL is connected to a Data bus via the BLSelection Circuit. In one embodiment, each bit line has its own BLselection circuit, and many bit lines can be connected to a multi-lineData bus.

The Data bus, which includes a capacitance 27766, is connected to nodeGSB via transistor 27764, which has its gate connected to ½ Vset. NodeGSB is connected to comparator 27780, which operates like comparator27720 of FIG. 25. The output of comparator 27780 is provided to thecontrol logic for the memory system. Transistor 27760, which isconnected to −VRR and to GSB, receives a pulse (PG Pulse). During apulse, GSB is floating. Between pulses, −VRR is used to charge GSB,which charges the Data bus. Based on the selection signals XCQ<3:0> and“decoder out” the BL Selection Circuit shares the charge on the data buswith the selected bit line in order to SET memory cell 27750 asdiscussed above.

The BL Selection Circuit of FIG. 27 includes transistor 27768,transistor 27770, inverter 27772, pass gate 27774 and pass gate 27776.Circle 27778 provides the details (four internal transistors andinverter) of pass gates 27774 and 27776. The pass gates have an input(i), inverted output (o), output (shown by an arrow), top node (t) andbottom node (b). If the input (i) is a positive voltage, the outputreceives the signal from the bottom node (b). If the input (i) is anegative or zero voltage, the output receives the signal from the topnode (t). Pass gate 27776 receives PGN Pulse which is the inversion ofthe pulse received by transistor 27760. During a pulse (positivevoltage), the appropriate one of XCQ<3:0>, which is input at the bottomnode of pass gate 27776, is provided at the output of pass gate 27776and transferred to the output of pass gate 27774 if “decoder out” isalso selecting the bit line with a positive voltage. The appropriate oneof XCQ<3:0> will be at 0V for the selected bit line and at −VRR for theunselected bit line. When the gate of transistor 27768 receives −VRR, itcuts off the bit line from the Data bus. When the gate of transistor27768 receives 0V, it shares the charge on the Data bus with the bitline. Note that the gate voltage (0V) of transistor 27768 can be set tocontrol the transient current by trim-options.

Between pulses input to pass gate 27776, −VRR will be transferred to theoutput of pass gate 27776 and to the output of pass gate 27774, which isthen provided to the gate of transistor 27768 to cut off the bit linefrom the Data bus. If XCQ<3:0> or “decoder out” is also selecting thebit line, then −VRR will be passed to the gate of transistor 27768 tocut off the bit line from the Data bus.

Referring to FIG. 28, a flow chart describing the operation of thecircuit of FIG. 27 in accordance with a representative embodiment isshown. In step 28788, the selected word line is driven to ½ Vset. Instep 28790, node GSB and the data bus are charged, as explained above bytransferring −VRR to node GSB between pulses of PG Pulses. −VRR ishigher in magnitude than the desired bit line voltage −½ Vset by acapacitive charge sharing ratio. In step 28792, the charge on the databus is shared with the bit line by connecting the bit line to the databus using the BL Selection circuit, as described above. In step 28794,the bit line is cut off from the data bus, thereby floating the bitline. As a result, the bit line discharges through memory cell 27750 instep 28796. In some embodiments, one iteration of the process of FIG. 28is enough to SET the memory cell. In other embodiments, multipleiterations are needed to SET the memory cell. Advantageously, thecircuits of FIGS. 25 and 27 limit the amount of electrical charge in theSET operation rather then the SET current.

Temperature Control of Biasing Voltage

Referring to FIG. 29, a current-voltage (I-V) graph of a diode of amemory cell at various temperatures in accordance with a representativeembodiment is shown. The graph shows the absolute current (A) versusvoltage (V) for a diode of a memory cell in a memory array. Plot 2910shows the current of the diode in reverse bias and in forward bias at 85degrees Celsius. Plot 2920 shows the current of the diode in reversebias and in forward bias at 25 degrees Celsius. Plot 2930 shows the loadline for a carbon storage element in an OFF state at 85 degrees Celsius,where the reverse bias setting mode is going to be used. Plot 2940 showsthe load line for a carbon storage element in an OFF state at 25 degreesCelsius, where the reverse bias setting mode is going to be used.

The reverse SET method for a resistivity switching element can include atemperature sensing circuit, on chip, which is used to control the Vsetbias level such that a higher bias is applied at colder temperaturesthan at higher temperatures. As illustrated by plots 2910 and 2920 thecurrent level flowing through the cell as SET starts, in reverse bias,is more consistent from the cold temperature operation to the hightemperature operation. The diode current changes about 10 times and Vsetchanges about 1 to 2 volts for an about 60 degree temperaturedifference. Thus, the setting voltage selected based on the sensedtemperature comprises increasing a magnitude of the setting voltage in arange of about 1 volt to 2 volts when the sensed temperature decreases,relatively, about 55 to 65 degrees Celsius. Both −½ Vset and +½ Vset canbe varied if the bias is split as in FIG. 22. In addition, the SETvoltage can also be adjusted for sets and resets where a forward bias isused (i.e., the SET operation may be conducted with forward or reversebias, and the RESET operation may be conducted with forward or reversebias) and where the storage element is any kind of resistivity storageelement (e.g., an antifuse dielectric, a fuse, a polysilicon memoryeffect material, a metal oxide or switchable complex metal oxidematerial, a carbon nanotube material, a graphene switchable resistancematerial, a phase change material, a conductive bridge element, anelectrolyte switching material, a switchable polymer material, or acarbon resistivity switching material). Various control circuits andmethods can be used to adjust Vset based on the temperature sensed.Advantageously, the biasing voltage, in particular a setting voltage,can be controlled precisely under varying temperature conditions.

Referring to FIG. 30, a circuit of a temperature-based algorithmic biasselector in accordance with a representative embodiment is shown. Aselector circuit 3000 includes an analog temperature sensor 3010, analgorithm storage/processing module 3020, and a Vset generator 3030. Theanalog temperature sensor 3010 senses a temperature of at least onememory cell or a temperature of a chip which includes the at least onememory cell. The at least one memory cell comprises a steering elementand a storage element. The analog temperature sensor 3010 can be locatedon the same die as the at least one memory cell or the analogtemperature sensor 3010 can be, for example, integrated into the samepackaging, assembly, or device as the at least one memory cell. Theanalog temperature sensor 3010 provides a temperature signal toalgorithm module 3020.

Algorithm module 3020 selects a setting voltage based on the sensedtemperature represented by the temperature signal. The algorithm module3020 includes a temperature-setting voltage model such as in FIG. 29.For example, the algorithm module 3020 includes a stored value slope ofa plot Vset variation as a function temperature. Alternatively, moresophisticated temperature-setting voltage models can be used. Thealgorithm module 3020 provides a Vset reference to the Vset generator3030. The Vset reference can be analog or digital.

The Vset generator 3030 provides the setting voltage to the at least onememory cell. In one embodiment, the steering element is reverse biasedand the storage element is SET to a lower resistivity state. Vsetgenerator 3030 can include a comparator 3040. The comparator 3040compares the Vset reference to the setting voltage and adjusts thesetting voltage accordingly.

Referring to FIG. 31, a circuit of a temperature-based look-up biasselector in accordance with an alternative representative embodiment isshown. A selector circuit 3100 includes a digital temperature sensor3110, a look-up table (LUT) module 3120, and a Vset generator 3130. Thedigital temperature sensor 3110 senses a temperature of at least onememory cell or a chip comprising the at least one memory cell. The atleast one memory cell comprises a steering element and a storageelement. The digital temperature sensor 3110 can be located on the samedie as the at least one memory cell or the digital temperature sensor3110 can be, for example, integrated into the same packaging, assembly,or device as the at least one memory cell. The digital temperaturesensor 3110 provides a digital temperature signal to look-up tablemodule 3120.

Look-up table module 3120 selects a setting voltage based on the sensedtemperature represented by the temperature signal. The look-up tablemodule 3120 includes a stored data table with setting voltages forvarious temperatures, such as a data table based on a plot shown in FIG.29. For example, the look-up table module 3120 includes a first Vsetreference for a memory cell at 85 degrees Celsius and a second Vsetreference for a memory cell at 25 degrees Celsius. In addition, thelook-up table module 3120 can include tables for various diodes andstorage element types. The look-up table module 3120 provides a Vsetreference to the Vset generator 3130. The Vset reference can be analogor digital.

The Vset generator 3130 provides the setting voltage to the at least onememory cell. In one embodiment, the steering element is reverse biasedand the storage element is SET to a lower resistivity state. Vsetgenerator 3130 can compare the Vset reference to the setting voltage andadjust the setting voltage accordingly. Thus, in the embodiments ofFIGS. 30 and 31, the setting voltage selected based on the sensedtemperature includes calculating the setting voltage based on analgorithm which is a function the sensed temperature or matching thesensed temperature to a setting voltage value in a look-up table.

The foregoing description of the exemplary embodiments have beenpresented for purposes of illustration and of description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and modifications and variations are possible in light of theabove teachings or may be acquired from practice of the invention. Forexample, the described exemplary embodiments focused on one layer ofmemory cells. The present invention, however, is not limited to onelayer. Those skilled in the art will recognize that the device andmethods of the present invention may be practiced using multiple levelsof memory cells that are mirrored, half mirrored, or have separateX-line and Y line layers as is well known in the art of threedimensional memory arrays. In a half mirrored arrangement the Y-line isshared between two levels of memory cells. The Y-line has diode layersboth above and below. Additionally, types of diodes and resistivityswitching materials may be changed without deviating from the spirit ofthe invention. Similarly, various types and numbers of diodes may beemployed such as distributed diodes. Likewise, the device and methods ofthe present invention may be practiced using other passive elementmemory systems. Consequently, a wide variety of current-voltage profilesare possible. Hence, a switching point voltage range of −5V to −15 isnot limiting. The embodiments were chosen and described in order toexplain the principles of the invention and as practical applications ofthe invention to enable one skilled in the art to utilize the inventionin various embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

While the invention has been largely described with respect to theembodiments set forth above, the invention is not necessarily limited tothese embodiments. For example, the instant invention can also beapplied to three-dimensional memory arrays configured as a plurality oflevels, where word lines and/or bit lines are shared between levels,including, but not limited to: (1) the memory described in U.S. Pat. No.6,034,882 issued on Mar. 7, 2000 and U.S. Pat. No. 6,185,122 issued onFeb. 6, 2001, to Mark G. Johnson, et al., both commonly assignedherewith; (2) the memory array described in U.S. patent application Ser.No. 09/560,626 filed on Apr. 28, 2000, in the name of N. Johan Knall andcommonly assigned herewith; (3) the memory array described in U.S.patent application Ser. No. 09/814,727 filed on Mar. 21, 2001, in thename of N. Johan Knall and Mark G. Johnson and commonly assignedherewith; The memory described in “Three-Dimensional Memory ArrayIncorporating Serial Chain Diode Stack” by Kleveland, et al, U.S. patentapplication Ser. No. 09/897,705, filed on Jun. 29, 2001; the memorydescribed in “Word Line Arrangement Having Multi-Layer Word LineSegments for Three-Dimensional Memory Array,” referenced above; and thememory described in U.S. patent application Ser. No. 10/185,508 byCleeves, filed Jun. 27, 2002, entitled “Three Dimensional Memory”, eachof which is hereby incorporated by reference.

As used herein, a passive element memory array includes a plurality of2-terminal memory cells, each connected between an associated X-line andan associated Y-line. Such a memory array may be a two-dimensional(planar) array or may be a three-dimensional array having more than oneplane of memory cells. Each such memory cell has a non-linearconductivity in which the current in a reverse direction (i.e., fromcathode to anode) is lower than the current in a forward direction.Application of a voltage from anode to cathode greater than aprogramming level changes the conductivity of the memory cell. Theconductivity may decrease when the memory cell incorporates a fusetechnology, or may increase when the memory cell incorporates anantifuse technology. A passive element memory array is not necessarily aone-time programmable (i.e., write once) memory array. The memory cellmay incorporate a reprogrammable memory material for which theconductivity may decrease or increase after application of a suitableelectrical pulse.

Such passive element memory cells may generally be viewed as having acurrent steering element directing current in a direction and anothercomponent which is capable of changing its state (e.g., a fuse, anantifuse, a capacitor, a resistive element, etc.). In certain preferredembodiments of the present invention, the memory element is a diode-likestructure having a p+ region separated from an n− region by an antifuseelement. When the antifuse element is programmed, the p+ region iselectrically connected to the n− region and forms a diode. Theprogramming state of the memory element can be read by sensing currentflow or voltage drop when the memory element is selected. In an organicPEMA embodiment, the memory element is a diode-like structure having ananode region separated from a cathode region by an organic materiallayer whose conductivity changes as electrons are injected into thelayer.

Preferably, the memory cells are comprised of semiconductor materials,as described in U.S. Pat. No. 6,034,882 to Johnson et al., U.S. Pat. No.5,835,396 to Zhang, U.S. patent application Ser. No. 09/560,626 byKnall, and U.S. patent application Ser. No. 09/638,428 by Johnson, eachof which are hereby incorporated by reference. Other types of memoryarrays that are stackable over support circuits, such as MRAM andorganic passive element arrays, can also be used. MRAM (magnetoresistiverandom access memory) is based on magnetic memory elements, such as amagnetic tunnel junction (MTJ). MRAM technology is described in “A 2556kb 3.0V ITIMTJ Nonvolatile Magnetoresistive RAM” by Peter K. Naji etal., published in the Digest of Technical Papers of the 2001 IEEEInternational Solid-State Circuits Conference, ISSCC 2001/Session7/Technology Directions: Advanced Technologies/7.6, Feb. 6, 2001 andpages 94-95, 404-405 of ISSCC 2001 Visual Supplement, both of which arehereby incorporated by reference. Certain passive element memory cellsincorporate layers of organic materials including at least one layerthat has a diode-like characteristic conduction and at least one organicmaterial that changes conductivity with the application of an electricfield. U.S. Pat. No. 6,055,180 to Gudensen et al. describes organicpassive element arrays and is also hereby incorporated by reference.Memory cells comprising materials such as phase-change materials andamorphous solids can also be used. See U.S. Pat. No. 5,751,012 toWolstenholme et al. and U.S. Pat. No. 4,646,266 to Ovshinsky et al.,both of which are hereby incorporated by reference. Memory cellscomprising resistance change materials including transition metaloxides, as described in more detail in U.S. patent application Ser. No.11/287,452 by Herner, et al. which is hereby incorporated by reference,carbon nanotube layers, which may be formed as described in US PatentPub 20050269553 Sen, Rahul, et al. which is hereby incorporated byreference, and amorphous, polycrystalline or microcrystalline carbonlayers can also be used.

Based upon the teachings of this disclosure, it is expected that one ofordinary skill in the art will be readily able to practice the presentinvention. The descriptions of the various embodiments provided hereinare believed to provide ample insight and details of the presentinvention to enable one of ordinary skill to practice the invention.Although certain supporting circuits (e.g., decoders, sensing circuits,multiplexers, input/output buffers, etc.) are not specificallydescribed, such circuits are well known, and no particular advantage isafforded by specific variations of such circuits in the context ofpracticing this invention. Moreover, it is believed that one of ordinaryskill in the art, equipped with the teaching of this disclosure, will beable to carry out the invention, including implementing various controlcircuits inferred but not specifically described herein, using wellknown circuit techniques and without undue experimentation. Nonetheless,additional details of bias conditions, bias circuits, and layer decodercircuits particularly suitable for a three-dimensional memory array ofwrite-once anti-fuse passive element memory cells are described in U.S.Pat. No. 6,618,295, entitled “Method and Apparatus for Biasing Selectedand Unselected Array Lines When Writing a Memory Array”, by Roy E.Scheuerlein, filed on Jun. 29, 2001, and in “Three-Dimensional MemoryArray Incorporating Serial Chain Diode Stack” by Kleveland, et al, U.S.patent application Ser. No. 09/897,705, filed on Jun. 29, 2001, whichare both hereby incorporated by reference in their entirety.

In the above description, an array line is generally shared by twolevels of the memory array (i.e., memory planes). Alternatively, amemory array may be fabricated having two conductors for each plane thatare not shared with other planes. A dielectric layer may be used toseparate each such memory level.

Word lines may also be referred to as row lines or X-lines, and bitlines may also be referred to as column lines or Y-lines. Thedistinction between “word” lines and “bit” lines may carry certainconnotations to those skilled in the art. When reading a memory array,it is assumed by some practitioners that word lines are “driven” and bitlines are “sensed.” Moreover, the memory organization (e.g., data buswidth, number of bits simultaneously read during an operation, etc.) mayhave some association with viewing one set of the two array lines morealigned with data “bits” rather than data “words.” Neither connotationis necessarily intended in this description.

The directionality of X-lines (e.g., which may be shown horizontally)and Y-lines (e.g., which may be shown vertically) is merely convenientfor ease of description of the two groups of crossing lines in thearray. While X-lines are usually orthogonal to Y-lines, such is notnecessarily implied by such terminology. Moreover, the word and bitorganization of a memory array may also be easily reversed, havingY-lines organized as word lines and X-lines organized as bit lines. Asan additional example, portions of an array may correspond to differentoutput bits of given word. Such various array organizations andconfigurations are well known in the art, and the invention in intendedto comprehend a wide variety of such variations.

The embodiments described may refer to a selected word line being drivento a voltage and a selected bit line being sensed in a read mode, andmemory cell anode terminals connected to word lines and cathodeterminals connected to bit lines, but other embodiments are specificallycontemplated. For example, in a three-dimensional (i.e., multi-level)memory array, an adjacent memory plane may be connected similarly (e.g.,a back-to-back diode stack memory array as described in U.S. Pat. No.6,034,882 to Johnson, et al., referred to above) so that the anodeterminals are connected to bit lines and the cathode terminals to wordlines, or may reverse the directionality of memory cells in the adjacentplane (e.g., a serial chain diode stack memory array as described inU.S. patent application Ser. No. 09/897,705 by Kleveland, et al.,referred to above). Consequently, the designations herein of X-lines,word lines, and row lines, and of Y-lines, bit lines, and column linesare illustrative of the various embodiments but should not be viewed ina restrictive sense, but rather a more general sense. For example,sensing circuits may be coupled to word lines rather than bit lines, ormay be used for both word lines and bit lines, when sensing a current ina word line rather than in a bit line. For example, it should beappreciated that the designations X-line and Y-line for various arraylines of a memory array on a serial chain diode stack do not necessarilyimply which terminal of the memory cells (i.e., anode or cathode) iscoupled to the particular line, as with a back-to-back diode stack. AnX-line may be coupled to the anode terminal of memory cells in oneassociated memory plane, and may be coupled to the cathode terminal ofmemory cells in an adjacent memory plane.

Integrated circuits incorporating a memory array usually subdivide thearray into a sometimes large number of smaller arrays, also sometimesknown as subarrays. As used herein, an array is a contiguous group ofmemory cells having contiguous word and bit lines generally unbroken bydecoders, drivers, sense amplifiers, and input/output circuits. Anintegrated circuit including a memory array may have one array, morethan one array, or even a large number of arrays. An used herein, anintegrated circuit memory array is a monolithic integrated circuitstructure, rather than more than one integrated circuit device packagedtogether or in close proximity, or die-bonded together.

The foregoing details description has described only a few of the manypossible implementations of the present invention. For this reason, thisdetailed description is intended by way of illustration, and not by wayof limitations. Variations and modifications of the embodimentsdisclosed herein may be made based on the description set forth herein,without departing from the scope and spirit of the invention. It is onlythe following claims, including all equivalents, that are intended todefine the scope of this invention. All patents and patent applicationsmentioned herein are incorporated by reference in their entirety.

What is claimed is:
 1. A method of programming a nonvolatile memory cellcomprising a diode steering element in series with a carbon storageelement, the method comprising: providing a first voltage to thenonvolatile memory cell, such that the diode steering element is reversebiased and the carbon storage element is set to a lower resistivitystate, wherein the carbon storage element comprises a polycrystallinecarbon resistivity switching material.
 2. The method of claim 1, furthercomprising: providing a second voltage to the nonvolatile memory cell,such that the diode steering element is forward biased and the carbonstorage element is reset to a higher resistivity state.
 3. The method ofclaim 2, wherein a magnitude of a second current associated with thesecond voltage is greater than a magnitude of a first current associatedwith the first voltage and the carbon storage element is set and reset aplurality of times.
 4. The method of claim 1, wherein providing thefirst voltage to the nonvolatile memory cell comprises providing avoltage pulse to the nonvolatile memory cell.
 5. The method of claim 1,wherein providing the first voltage to the nonvolatile memory cellcomprises at least one of charging a bit line associated with thenonvolatile memory cell or discharging a capacitive storage elementacross the memory cell wherein the capacitive storage element comprisesa capacitor driving the bit line associated with the nonvolatile memorycell.
 6. The method of claim 1, wherein: the diode steering elementcomprises a p-n semiconductor diode, a p-i-n semiconductor diode, or apunch-through diode.
 7. The method of claim 1, wherein: the carbonstorage element and the diode steering element comprise a pillar shapedcell located over a lower electrode and under an upper electrode; andthe diode steering element is located over or under the carbon storageelement.
 8. The method of claim 2, wherein: when the carbon storageelement is set to the lower resistivity state, conductive linkages areformed between portions of a carbon material of the carbon storageelement; and when the carbon storage element is reset to the higherresistivity state, conductive linkages are broken between the portionsof the carbon material of the carbon storage element.
 9. The method ofclaim 1, further comprising: programming a nonvolatile memory cellarray, the nonvolatile memory cell array comprising a plurality of Xlines, a plurality of Y lines, and a plurality of memory cells, eachmemory cell comprising the diode steering element in series with thecarbon storage element and each memory cell electrically connectedbetween one of the plurality of X lines and one of the plurality of Ylines, wherein providing the first voltage comprises: driving a selectedX line of the plurality of X lines associated with at least one selectedmemory cell of the plurality of memory cells to a X line select voltage;driving at least one selected Y line of the plurality of Y linesassociated with the at least one selected memory cell to a Y line selectvoltage; wherein the X line select voltage and the Y line select voltageare configured such that the diode steering element of the at least oneselected memory cell is reverse biased; isolating the at least oneselected Y line of the plurality of Y lines before the carbon storageelement of the at least one selected memory cell is completely set to alower resistivity state; and holding the at least one selected Y line inan isolated state until the carbon storage element of the at least oneselected memory cell is completely set to the lower resistivity state.10. The method of claim 9, wherein: driving the selected X line to a Xline select voltage, driving at least one selected Y line to the Y lineselect voltage, and isolating the at least one selected Y line cause anamount of charge to form on the at least one selected Y line; and theamount of current available to completely set the carbon storage elementof the at least one selected memory cell to the lower resistivity stateis limited by the amount of the charge.
 11. The method of claim 9,wherein isolating the at least one selected Y line of the plurality of Ylines comprises floating the at least one selected Y line.
 12. Themethod of claim 11, wherein one of a plurality of MOSFETs is connectedto each one of the plurality of Y lines, and floating the at least oneselected Y line comprises turning off the MOSFET transistor associatedwith the at least one selected Y line.
 13. The method of claim 12,wherein the MOSFET transistor is a NMOS transistor isolated in a p-typesemiconductor well biased at a negative well voltage that is equal to orless than a voltage applied to a source or gate of the NMOS transistor.14. The method of claim 9, wherein the at least one selected Y line ofthe plurality of Y lines is electrically connected to a data bus drivenby a capacitive discharge circuit, and the data bus is isolated from adriving circuitry of the nonvolatile memory cell array.
 15. The methodof claim 9, further comprising: driving the selected X line and the atleast one selected Y line such that the diode steering element of the atleast one selected memory cell is forward biased and the carbon storageelement of the at least one selected memory cell is reset to a higherresistivity state.
 16. The method of claim 9, wherein the diode steeringelement comprises a p-n semiconductor diode, a p-i-n semiconductordiode, or a punch-through diode.
 17. The method of claim 9, wherein eachof the plurality of memory cells comprises a pillar shaped cell locatedbetween one of the plurality of X lines and one of the plurality of Ylines; and the diode steering element is located over or under thecarbon storage element.
 18. The method of claim 1, further comprising:sensing a temperature of at least one memory cell or a chip comprisingthe at least one memory cell, the at least one memory cell comprisingthe diode steering element and the carbon storage element; selecting asetting voltage based on the sensed temperature; and providing the firstvoltage comprising the setting voltage to the at least one memory cell,such that the steering element is reverse biased and the storage elementis set to the lower resistivity state.
 19. The method of claim 18,further comprising providing a resetting voltage to the at least onememory cell, such that the steering element is forward biased and thestorage element is reset to a higher resistivity state.
 20. The methodof claim 18, wherein providing the setting voltage to the at least onememory cell comprises providing a voltage pulse to the at least onememory cell.
 21. The method of claim 18, wherein: the sensed temperaturecomprises a first temperature or a second temperature; a first settingvoltage is associated with the first temperature a second settingvoltage is associated with the second temperature; and when the secondtemperature is lower than the first temperature, a magnitude of thesecond voltage is greater than a magnitude of the first voltage.
 22. Themethod of claim 18, wherein selecting the setting voltage based on thesensed temperature comprises increasing a magnitude of the settingvoltage in a range of about 1 Volt to 2 Volts when the sensedtemperature decreases, relatively, about 55 to 65 degrees Celsius. 23.The method of claim 18, wherein selecting the setting voltage based onthe sensed temperature comprises calculating the setting voltage basedon an algorithm which is a function the sensed temperature or matchingthe sensed temperature to a setting voltage value in a look-up table.24. The method of claim 18, wherein the steering element comprises a p-nsemiconductor diode, a p-i-n semiconductor diode, or a punch-throughdiode.
 25. The method of claim 18, wherein: the storage element and thesteering element comprise a pillar shaped cell located over a lowerelectrode and under an upper electrode; the steering element is locatedover or under the storage element; and the storage element and thesteering element are electrically connected in series.
 26. A method ofprogramming a nonvolatile memory cell comprising a diode steeringelement in series with a carbon storage element, the method comprising:providing a first voltage to the nonvolatile memory cell, such that thediode steering element is reverse biased and the carbon storage elementis set to a lower resistivity state, wherein the carbon storage elementcomprises a graphene switchable resistance material.